Tens of millions of tons of phosphorus-containing minerals, such as apatite, rock phosphate, etc., are processed for producing phosphorus-containing fertilizers. Typically, the processing is carried out by treating these natural materials with concentrated nitric or sulphuric acid. During the treatment with sulphuric acid, apatite is decomposed with precipitation of calcium sulphate and formation of phosphoric acid solution. In this case, the main waste product is phosphogypsum (calcium sulphate contaminated with impurities of P2O5, F, Fe, Al, Sr, REM) which comprises most of the rare-earth metals contained in apatite. For example, apatite of the Kola Peninsula contains up to 1% of rare-earth metals, the 70 to 100% of which are precipitated with calcium sulphate at apatite processing with sulphuric acid. Phosphogypsum constitutes whole mountains around the plants for processing of apatite. Every year millions of tons of phosphogypsum containing about 0.5% REM in terms of oxides, which currently can not be extracted from it, are sent to dumps. Furthermore, the presence of such dumps containing toxic compounds including fluorine is an environmental problem. In this regard, numerous research projects have been conducted to develop processing technology to extract REM and remove toxic components.
A method for extracting rare-earth elements from phosphogypsum by treatment with nitric acid and subsequent extraction of rare earth elements (REE) by phosphine oxide is described in Martynova I. N. et al. Research of distribution of REE in the course of extraction from acidic nitrate-phosphate solutions. Collected articles “Processing and physico-chemical properties of compounds of rare elements. Apatity, 1984, pp. 6-8 (Rus). The disadvantage of this method is the need for expensive trialkyl phosphine oxide and the impossibility of complete liquid-phase removal of REE from the organic phase. Furthermore, because of the high loss of trialkyl phosphine oxide with the aqueous phase, this method is uneconomical and requires additional facilities for trialkyl phosphine oxide utilization.
Nitric acid extraction technology for isolation of rare earth elements from apatite, giving up to 85% release in a solution also containing phosphorus and fluorine is described in (Kosynkin V. D. et al. “Condition and perspective of rare earth industry in Russia”, “Metals” (rus), No. 1, 2001). The disadvantage of this method is the impossibility of using process solutions in a closed loop and the subsequent low recovery of REM in the process in closed loop.
A method for extracting rare earth elements from phosphogypsum is described in PCT publication WO2011008137. The method involves the acid extraction of rare earth element compounds from phosphogypsum using a solution consisting of a mixture of sulphuric acid and nitric acid in a ratio of 3.2:1.2 with a concentration of 1-3 wt. % and a liquid to solid ratio of 4:5 over a period of 8-12 minutes, while the extraction suspension is agitated and subjected to a hydroacoustic effect. The insoluble gypsum residue is then separated from the extraction suspension and the rare earth element compounds are recovered from the extraction solution by cation exchange sorption with the extraction solution being passed through a cation exchange filter. The main disadvantages of this method are not sufficiently high enough degree of extraction of rare earth metals (up to 85%) with the high cost ion exchanger, long duration of the process and large material flows.
A method of recovering rare-earth elements from phosphogypsum disclosed in RU patent No. 2293781. The method comprises treatment of phosphogypsum with sulphuric acid solution to recover rare-earth elements into solution, separation of gypsum precipitate, increasing of oversaturation rate of the solution in terms of rare-earth elements to crystalize rare-earth metal concentrate, and separation of the concentrate from mother liquor followed by concentrate processing. Phosphogypsum is treated with 22-30% sulphuric acid solution at liquids-to-solids ratio 1.8-2.2 during 20-30 min to prevent spontaneous crystallization of rare-earth element concentrate in solution before insoluble precipitate is separated. An increase of the oversaturation rate of the solution is achieved by means of providing sodium concentration 0.4-1.2 g/L. The disadvantage of this method is the use of additional reagents, high acid concentrations and significant amounts thereof, a large number of basic technological operations with incomplete extraction of rare earth elements (up to 71.4%) and the overall complexity of the process.